The highchair 400′ in FIG. 1 was disclosed in U.S. Pat. No. 4,768,825. When the highchair 400′ is collapsed, its front legs 11′ and rear legs 12′ cannot be pivotally rotated to be in a line. Also, its seat assembly 13′ and backrest 14′ cannot be moved to be in a line with the front legs 11′ and rear legs 12′. Therefore, the volume of the collapsed highchair 400′ is still large. Furthermore, the locking device used for keeping the highchair 400′ in a stretched state is difficult to be operated and is unstable.